1. Field of the Invention
The present invention relates to a motor control apparatus, in particular to a motor control apparatus having a function of determining whether or not a machine protecting operation should be started.
2. Description of the Related Art
Motor control apparatuses for driving motors in a machine tool, forging press machine, injection molding machine, robot or the like use a rectifier for converting AC power from a three-phase AC input power supply into DC power and an inverter for converting DC power output from the rectifier into AC power for motor drive.
In a motor control apparatus of the above kind, if a power failure occurs on the AC power supply side of the rectifier and resultantly the input power voltage lowers, normal motor operation cannot continue, causing some faults in the process using the motor control apparatus. In order to deal with this, the motor control apparatus needs to have a power failure detecting device on the AC power supply side so as to be able to perform a protecting operation for avoiding or minimizing the faults when a power failure is detected.
To detect a power failure, for example there is a known method of detecting a power failure by transforming the components of the three-phase AC input power supply into a voltage vector on the 2-phase coordinate system, calculating the amplitude of the vector to determine the amplitude of the power supply voltage, and determining the occurrence of a power failure when the calculated value has continued to be lower than a reference voltage level for a predetermined reference period of time (e.g., Japanese Patent Application Laid-open No. 2006-14546(JP 2006-14546 A)).
However, if the voltage of the DC link, i.e., the DC output side of the rectifier, is kept as is even at the time of a power failure, the motor can continue normal drive. At most of momentary power failures (momentary voltage drops) the voltage of the DC link will not drop thanks to the effect of the accumulated electricity in the smoothing capacitor of the rectifier. Nevertheless, in the prior art technology, once a momentary power failure occurs, the system detects the power failure and takes a protecting action such as axis stoppage and axis retraction even if the motor can continue running. This leads to a drop of working efficiency of the machine.
A power failure detecting method based on the amplitude value of the power supply voltage in a conventional motor control apparatus will be described. FIG. 1 is a configurational diagram of a conventional electric control apparatus. In a conventional motor control apparatus 1000, AC power is supplied from three-phase AC power supply 10 to a rectifier 110 via an AC reactor 190, and the rectifier 110 converts AC power into DC power. The DC power output from the rectifier 110 is smoothed by a smoothing capacitor 120 provided in a DC link 200 and then supplied to an inverter 130. The inverter 130 converts the DC power into AC power to drive a motor 30.
As shown in FIG. 1, an AC voltage detector 140 detects the voltage of the three-phase AC power supply 10, and a voltage amplitude calculator 150 calculates the amplitude value. A power failure detector 160 determines that a power failure is occurring in the three-phase AC power supply 10 when the amplitude value has been continuously lower than a reference voltage level for a predetermined period of time.
A power failure occurring in the three-phase AC power supply 10 does not directly mean an instant cessation of normal operation of the motor 30. The motor 30 can continue normal operation as long as the energy supplied to the inverter 130 (=the energy at the DC link on the DC output side of the rectifier 110) is retained.
It is true that if an event of a momentary power failure coincides with an event of acceleration or deceleration in the motor, in other words, an event at which the motor's output is maximized, there is a risk that the DC voltage will drop. However, occurrence of such a situation is extremely rare. Accordingly, in most momentary power failures, due to the effect of the accumulated electricity in the smoothing capacitor of the rectifier, the motor can continue normal operation without being affected by a sharp drop of the DC link voltage.
However, in the conventional power failure detecting method based on the amplitude value of the power supply voltage, detection of whether or not the power supply is in a state of power failure is performed regardless of the state of the DC link. As a result, a momentary power outage for which no protecting action is needed is also detected as a power failure so that the machine implements a protecting operation.
If the machine performs a protecting operation, the machine processing stops though no work and tools will be damaged. Therefore, if a machine protecting operation is performed every time a momentary power outage takes place, there is a problem that work efficiency of the machine degrades.